Repair of DNA treated with chemical carcinogens or with gamma-irradiation is under investigation. The major focus is on enzymes which recognize these forms of damage. Endonuclease II of E. coli will be purified to homogeneity and its exact mechanism, with regard to the N-glycosidase and the phosphodiester bond hydrolase, will be studied. The activity of several mammalian systems is also under investigation. Some of the aspects of radiation damage, such as the structure of altered base(s) which is (are) recognized by endonuclease II, will be investigated. BIBLIOGRAPHIC REFERENCES: Kirtikar, D.M., Kuebler, J.P., Dipple, A. and Goldthwait, D.A. (1976) Endonuclease II of E. coli and Related Enzymes. 6th International Symposium of the Princess Takamatsu Cancer Research Fund, p. 349-362. Kuebler, J. P. and Goldthwait, D.A. (1977) An Endonuclease from Calf Liver Specific for Apurinic Sites in DNA. Biochemistry 16, in press.